1. Field of the Invention
The present invention pertains to a method for driving a brushless DC motor, more particularly, to a method for driving a brushless DC motor, in which a stator of the motor selectively stops providing a magnetic field for a rotor based on the angular position of the rotor corresponding to the stator.
2. Description of the Prior Art
Motors are well known and widely used in electrical and electronic industries. A conventional motor utilizes windings as its internal rotor, in which two ends of the armature windings are continuously interchangeably coupled to external circuits through the rotating process of the rotor and thereby direction commutation for the current on the armature is ruled. Using this scheme for the motor rotation, motor brushes are apt to be worn away through mechanical rubbing against the rotating contacts. This not only causes increased impedance and bad contact with external circuits, but also gives off a spark when bad contact has occurred. In a brushless DC motor, windings are wound around the stators, and permanent magnets are used as rotors. Electronic circuits are applied instead to control current flow direction commutation through windings and thus the polarity distribution of the magnetic field is continuously interchanged. Through such a strategy, no contact switching is required, and mechanical contact attrition is effectively avoided.
The mechanism of driving the brushless DC motor is described in the flow chart of FIG. 1. A Hall sensor (or Hall IC) is adopted to sense the magnetic field rotor distribution (Step 101). According to the sensing information, a driving control signal is then generated (Step 102). The driving control signal is issued to determine the current""s direction on the stator windings. The magnetic field polarities induced by the current are then interchanged with the alternated current direction (Step 103). Since the induced magnetic field exerts a force on the permanent magnets of the rotor, the rotor is then rotated in a predetermined direction (Step 104).
The magnetic interactions between the rotor and the stator and the rotation mechanism of the brushless DC motor are illustrated in FIG. 2a to FIG. 2d. In FIG. 2a, the four arms 112, 114, 116 and 118 of the stator 110 points respectively to the four joints of the four magnetic arcs 122, 124, 126 and 128 of the rotor 120, wherein every two neighboring arcs have a joint between them. A Hall sensor 130 is located on the angle bisector of the arms 112 and 114 and near the rotor 120. The Hall sensor 130 is capable of sensing the magnetic field irradiated from the S-polar magnetic arc 124. Based on the sensed information, a driving signal is generated to control the current""s direction on the windings and thus the polarities of the four magnetic arms, which are shown in FIG. 2a. Therefore, the magnetic field irradiated from the four arms of the stator will exert magnetic force on the magnetic arcs of the rotor. The force directions by the four arms of the stator on the rotor are approximately presented as the hollow arrows 131 to 138.
As an example of the N-polar magnetic arc 122, the N-polar magnetic arc 122 both receives an attractive force 132 by the arm 114 and a repulsive force 131 by the arm 112. The tangent components (the forces are tangent to the rotor) of the two forces 132 and 131 on the circular rotor create a resultant force, which will drive the rotor to rotate counterclockwise in an angular acceleration motion manner (the vector of the angular acceleration is perpendicular to and penetrates through the sheet). Similarly, the S-polar magnetic arc 124 both receives an attractive force 134 by the arm 116 and a repulsive force 133 by the arm 114. The tangent components of the two forces 134 and 133 on the circular create a resultant force, which will also drive the rotor to rotate counterclockwise in an angular acceleration motion manner. In addition, N-polar arc 126 and the S-polar arc 128 also receive the same magnetic interactive mechanism. Therefore, the motor rotor 120 will rotate counterclockwise, indicated as the direction of the arrow 139, about the central point (the joint of the four arms) of the stator (the joint of the four arms).
As the motor rotor 120 rotates counterclockwise from FIG. 2a to FIG. 2b, the magnetic arms 112, 114, 116 and 118 of the stator 110 point to the center of the four magnetic arcs of the rotor 120, respectively. Thus the four rotor magnetic arcs 122, 124, 126 and 128 will receive four centripetal magnetic forces, which points to the center of the stator as the hollow arrows 142, 144, 146 and 148 in FIG. 2b show, by the stator 110. Obviously, the tangent components of the magnetic force on the rotor 120 is zero, and thus the rotor 120 will not accelerate in a tangent direction. At this time, the rotor 120 will continue rotating counterclockwise owing to inertial mechanism. Now the Hall sensor is located near the joint and along the angle bisector of the S-polar 128 and N-polar 126 magnetic arcs, and thus senses a zero net magnetic field.
When the rotor 120 rotates counterclockwise to the angular position corresponding to the stator in FIG. 2c from that in FIG. 2b, the Hall sensor is slightly deviated from the position corresponding to the rotor 120 as compared to that in FIG. 2b and thus senses magnetic force from the N-polar magnetic arc 126. A driving control signal is in turn issued to change the direction of the current flowing though the stator windings, and thus the polarity commutation of the magnetic field induced by the direction changeover of the current is achieved. After the current""s direction changeover, the stator""s four arms polarities are depicted in FIG. 2c. In FIG. 2c, the arms 112 and 116 are S-polar, while the arms 114 and 118 are N-polar. Then, the rotor""s magnetic arcs are exerted, small amounts of tangent force by the stator""s arms, and the four magnetic arcs force receptive directions are indicated as the hollow arrows 152, 154, 156 and 158. Thus, the rotor 120 continues rotating counterclockwise in an accelerating manner.
When the rotor 120 rotates counterclockwise to the angular position corresponding to the stator in FIG. 2d from that in FIG. 2c, the rotor""s magnetic arcs polarity distribution and the stator""s magnetic arms is just the opposite to that in FIG. 2a. The repulsive force 131 and 135 in FIG. 2a is now replaced by the repulsive force 161 and 165 in FIG. 2d, wherein the repulsive force 161 and 165 are the same as the repulsive force 131 and 135 both in direction and quantity. The same magnetic mechanism happens to the repulsive forces 133 and 137 in FIG. 2a and 163 and 167 in FIG. 2d also. However, the attractive forces 134 and 138 in FIG. 2a are now replaced by the attractive force 164 and 168 in FIG. 2d, wherein the attractive forces 134 and 138 are the same as the attractive forces 164 and 168 both in direction and quantity.
According to the rotating mechanism described above, the rotor has the maximum angular acceleration for rotating in the case as FIG. 2a. Then, the angular acceleration gets smaller and smaller and then becomes zero when it corresponding to the angular position related to the stator in FIG. 2b. At that time, the angular positions of the four magnetic arcs of the rotor corresponding to the stator are called critical positions, and the angular position of the rotor is called a critical position.
In a case where the polarities of magnetic arms are kept constant, four arms of the stator generates tangent accelerations opposite to the rotating direction, after the rotor rotates across the critical position, and thus the rotating speed of the rotor is decreased. To achieve a desired continuous positive acceleration for the rotor, the polarities of the arms must change whenever the rotor reaches the critical position. Once the polarities of the arms are regularly interchanged, the rotor""s rotation, and thus the motor, is steadily maintained. Unfortunately, signals transmitted in electronic circuits undesirably have inherent delays. The driving control signal for changing the direction of the current flowing through the stator windings unexceptionally encounters this. When the Hall sensor acquires the rotor""s four magnetic arcs arrival information at the critical positions, a driving control signal for a current""s direction switching is then issued. Within the momentary interval for sensing of the Hall sensor and the driving control""s transmission signal based on the sensed information, the rotor""s magnetic arcs must have fast passed the critical positions. At this time, force exerted on the rotor opposite to the rotating direction is undesirably produced.
Once the reverse force is generated, the rotor""s reverse acceleration is followed. Under such a condition, the motor""s rotating efficiency is decreased because of the canceling out of the clockwise and counterclockwise tangent force. In addition, the reverse magnetic force will make the rotor""s rotation less smooth when working, and cause increased friction between machine parts. Moreover, the chattering and scraping phenomena within mechanical deviation tolerance will appeared, which will not only render the rotor to make significant noise, but shorten the lifetime of the motor.
In view of the drawbacks inhered in the conventional brushless DC motor that produces reverse interactive force and exerts force on the rotor when the rotor passes through the critical position due to delay of an electric control signal through wires on the circuit board. A need to drive the motor with high efficiency and low noise is thus created. To pursue this, the present invention discloses a method for driving a brushless DC motor. In the method, winding current is inactive on some intervals when a rotor""s four arcs are rotated slightly more or less than critical positions. This is done so that the rotor can rotate smoothly in a predetermined direction, during which time it suffers no affect by the reverse interactive force caused by the electric driving control signal""s delay time. Thus, the poor rotating efficiency encountered in prior art is significantly improved.
The method for driving the brushless DC motor is detailed below: Firstly, a Hall sensor is used to acquire the magnetic field""s information distribution of the motor rotor. Based on the information acquired, the driving control signal is successively generated. When the rotor is rotated to be within a critical area, the driving control signal is inactive and thus no magnetic field is produced by the stator. At this time, the rotor keeps rotating action inertial. However, when the rotor is not within the critical interval, the driving control signal is issued to drive the stator to produce a magnetic field in a conventional manner so that the motor rotates with an acceleration at the interval.